In modern printed circuit board technology, such as in surface mount technology, small electronic circuit components are attached electrically by soldering techniques to a printed circuit board. If the components are not properly mounted on the board, the board can then become rejected during the manufacturing process. Such a situation is very expensive, and of course, undesirable.
The rejects frequently occur as a result of the improper placement of solder paste deposits on the printed circuit board prior to the mounting of components thereon. In this regard, a metal screen process is employed to apply the solder paste to the board. In so doing, a larger or a smaller volume of solder paste may be applied inadvertently. If too little solder paste is applied, the electrical components do not have the proper electrical and mechanical bonding characteristics. If an excessive amount of solder paste is deposited, and heat applied, the solder paste flows beyond the electrical component lead and can form bridging or undesirable and unwanted short circuits, thereby leading to a rejection of the board.
Additionally, should the solder paste deposit be applied in an improper position on the surface of the board, rejections can also result. Thus, both the volume of the solder paste deposit and the positioning thereof are usually critical. Accurate solder paste positioning is especially important where very small electrical components having fine pitch leads, such as surface mount devices, are employed.
Therefore, it would be highly desirable to somehow be able to inspect both the volume and positioning of each individual solder paste pad on a printed circuit board. However, such an inspection must be made very quickly in a matter of seconds, in order not to slow down the throughput of the manufacturing process. For example, a three inch by five inch printed circuit board may have up to 600 small solder paste pads or deposits thereon, which all must be inspected for both volume and positioning in less than two seconds.
One attempted solution to verify the accuracy of solder paste deposit positions was tried, wherein a laser produces a small spot of high intensity light which was projected toward the surface of a printed circuit board. The intensity of the reflected spot was visualized by a camera to generate a signal in an attempt to verify the approximate height of the illuminated area relative to an expected height. However, as the amount of light reflected was extremely small, only a few such discrete inspections could be made per board within a reasonable amount of time, to determine whether solder paste deposits on the board were positioned within expected tolerance limits.
While such a system may permit a random sampling of a few solder paste deposits, it is unable to determine with accuracy the actual volume of the solder paste deposits within. Moreover, such an arrangement is only able to inspect a relatively few number of pads within a matter of a short time, such as two to three seconds.